1. Field of the Invention
This invention relates to a solder paste for soldering surface mount integrated circuit devices and surface mount discrete devices to circuit boards.
2. Related Art
Surface mount technology is emerging as an important new technique which rapidly is replacing dual-in-line pin (DIP) packaging for VISI devices. Surface mount integrated circuit devices can increase considerably the device density on a circuit board and they are effective in reducing the trace and lead lengths, thereby minimizing electrical noise problems.
Surface mount integrated circuit devices come in a variety of different package styles, including J leaded devices such as plastic leaded chip carriers (PLCCs) and leadless chip carriers (LCCs). These devices often are VLSI with high lead counts.
Traditional board soldering methods, such as wave soldering, applicable to DIPs, are more expensive when used with surface mount devices, hence, new board soldering equipment has been developed for surface mount devices. Typically, for large size boards, a solder paste reflow method is used with a vapor phase furnace. The solder paste usually is a mixture of a tin/lead solder alloy with resin and well-known solvents. The composition of the solder alloy typically is 63% tin and 37% lead, which is the tin/lead alloY composition with the lowest possible melting temperature. That is, 63% tin and 37% lead is the eutectic alloy of tin and lead. This alloy has a melting temperature of 183 degrees C. The solder paste is applied to all points (footpads) on a circuit board to which the leads of surface mount devices are to be soldered. The surface mount devices are then placed upon the circuit board which thereafter is positioned in a vapor phase furnace. In the furnace, the board, the surface mount device leads and the solder paste are rapidly heated to melt the solder to fuse the device leads to the footpads. The furnace temperature is 215 degrees C.
The present day solder paste reflow method is susceptible to producing a high proportion of open solder joints for J leaded integrated circuit devices, and weakened solder joints containing voids and cavities for most surface mount package styles.
An open solder joint between a device lead and a circuit board footpad occurs when most or all of the solder on the footpad wicks up along the length of the lead, leaving an insufficient quantity of solder at the pad to form a solder joint or fillet between the pad and lead. Solder wicking takes place when solder wetting forces on the lead are stronger than those on the footpad, causing the solder to be drawn to the lead surfaces more strongly than to the pad surface. This can happen when solder wetting occurs before or more strongly on the lead surfaces than it does on the footpad surface, and where the lead is sufficiently far from the surface of the pad.
High lead count, VLSI, surface mount devices are susceptible to lead aplanarity. Leads may be out of plane with each other by as much as several mils. Experience has shown that when a lead is farther than about one mil from the surface of a footpad, wicking sufficient to cause an open solder joint can take place. Also, it has been found that approximately five percent of the open solder joints which do occur, happen when lead/footpad separations are between one and two mils, and approximately ninety-five percent of the opens occur when separations are greater than two mils.
In addition to lead/footpad separations of from one to several mils, stronger solder wetting of the lead than the footpad is required for an open solder joint to occur. FIG. 1 shows the temperatures of a typical circuit board and of the leads of a J lead device after insertion into a vapor phase furnace. As seen in FIG. 1, the temperature of the device leads rises from 183 degrees C. (melting point of eutectic solder, and the temperature at which wetting begins) to the furnace temperature of 215 degrees C. more quickly than does the temperature of the circuit board (including the footpads mounted on the board). This occurs because the thermal mass of the circuit board is much greater than that of the leads. Since solder wetting occurs more quickly on hotter surfaces than on cooler ones, it is evident from FIG. 1 that solder wetting will occur more quickly on the lead surfaces than on the footpad surface, thus giving rise to stronger wetting forces attracting the solder to the lead surfaces than to the footpad surface. When the lead/footpad separation is sufficiently great, the wetting forces are dominant and sufficient wicking takes place to form an open solder joint.
Experience with the present day method of reflow soldering J lead devices in a vapor phase furnace has shown that the frequency of occurrence of open solder joints varies from approximately 250 to 2,000 per million solder joints attempted. 1,000 bad joints per million attempted gives a ratio of one bad joint to every 1,000 attempted. Many large modern circuit boards have several thousand joints per board, thus, it is possible to have a zero yield of perfect boards, leading to considerable costs in trouble shooting and rework.
The unwanted creation of voids and cavities in the solder joints between the contacts of surface mount integrated circuit devices and the footpads on the circuit board is another drawback of the present day method of solder reflow in a vapor phase furnace. Most conventional solder pastes outgas solvents and flux vehicles during reflow, or melting of the solder. Unfortunately, some of these gases continue to be produced after the surface layer of solder has melted, and since liquid solder has a very high surface tension which prevents the gases from escaping, the gases are trapped within the joint as voids. Investigation has shown that the voids weaken the solder joint, making it from ten to one thousand times more susceptible to fatigue and cracking, especially where leadless ceramic chip carriers are involved.